1. Field of the Invention
The present invention relates to a PET device (Positron Emission Tomography device) that coincidentally measures a pair of γ-rays emitted from an agent labeled with positron nuclides and performs a scan for obtaining a tomographic image at a site of interest.
To be specific, the present invention relates to a PET device used in combination with an X-ray CT (Computed Tomography) device.
2. Description of the Related Art
A PET (Positron Emission Tomography) test is a very useful technique for early detection of cancer. However, a PET image represents the accumulation and distribution state of nuclides, but does not show the location thereof in tissues within a human body. As a device for obtaining the shape of the tissues within the human body, there is a device in which a PET test and an X-ray CT test of imaging the distribution of X-ray absorption coefficients are used in combination. A method of executing a precise diagnosis by observing a fusion image composed by superimposing a PET image and an X-ray CT image by this device has been practically used (refer to Japanese Unexamined Patent Application Publication No. 2005-121530, for example).
The device in which the PET device and the X-ray CT device are combined has a top panel held on a bed so as to be movable horizontally, and a cylindrical PET gantry (a PET detector) and a CT gantry (a CT detector) through which a subject placed on the top panel can pass. In the order of proximity from the top panel, the CT gantry and the PET gantry are arranged close to each other. As the top panel moves, data of a PET image and an X-ray CT image of the same plane of the subject are acquired on each scan plane. After a PET image and an X-ray CT image are obtained through image processing by an image processor (not shown) composed of a computer and a memory, these images are superimposed and composed, and displayed as a fusion image on an image display.
Further, there is a conventional system known as SPECT (Single Photon Emission Computed Tomography), which is configured by a rotating part and a fixed part as in a CT device. With regard to this SPECT system, there is a technique of capturing an image by rotating a gantry (refer to Japanese Unexamined Patent Application Publication No. 2006-192286, for example). However, agents used for a PET image and a SPECT image are different from each other. Two γ-rays are generated from the agent used for a PET image. On the other hand, an image is generated with particles of only one γ-ray generated from the agent used for a SPECT image. Therefore, the SPECT system as a mechanism merely needs to detect particles of a γ-ray travelling in one direction, and there is no need to arrange γ-ray detectors at the facing positions. On the contrary, the PET device needs to detect particles of a pair of γ-rays travelling in opposing directions at 180°, and it is always necessary to arrange γ-ray detectors at the facing positions.
Besides, since a SPECT image has lower resolution than a PET image, the SPECT image is suitable for a measurement of the blood flow rate in the brain but hard to be used for a measurement of the distribution of glucose metabolism at a spot where cancer develops.
In the technique described in Japanese Unexamined Patent Application Publication No. 2005-121530, there is a certain distance between the PET detector and the CT detector as described above.
Therefore, there is a need to make the top panel protrude largely from the bed when performing a PET test, which causes the top panel to bend largely. When the top panel thus bends largely, the position of the subject is displaced from the center for capturing an image, and therefore, the quality of the PET image deteriorates. Accordingly, it is difficult to generate a PET image of favorable image quality.
The technique described in Japanese Unexamined Patent Application Publication No. 2006-192286 is a technique for the SPECT device. The SPECT device and the PET device are largely different in resolution, and the SPECT device does not have resolution required in the PET device. Therefore, it is difficult to implement the technique described in Japanese Unexamined Patent Application Publication No. 2006-192286 as it is in the PET device. Moreover, also in Japanese Unexamined Patent Application Publication No. 2006-192286, the SPECT device and the X-ray CT device are arranged in separate gantries. Therefore, it is difficult to reduce a difference in bend of the top panel between capturing images by the devices.
Further, since a conventional PET device is provided with γ-ray detectors on the entire circumference inside the cylindrical PET gantry, the cost is high. Furthermore, since a large number of detectors are arranged on the entire circumference inside the PET gantry, it is relatively difficult to manage the properties of the detectors uniformly.
Besides, it takes much time for an investigation when a failure occurs in the detectors.